INTERPLAY OF POSITIVE AND NEGATIVE REGULATORS IN TRANSCRIPTION INITIATION BY RNA-POLYMERASE-II HOLOENZYME

Activation of protein encoding genes involves recruitment of an RNA po lymerase II holoenzyme to promoters. Since the Srb4 subunit of the hol oenzyme is essential for expression of most class II genes and is a ta rget of at least one transcriptional activator, we reasoned that suppr essors of a temperature-sensitive mutation in Srb4 would identify othe r factors generally involved in regulation of gene expression, We repo rt here that MED6 and SRB6, both of which encode essential components of the holoenzyme, are among the dominant suppressors and that the pro ducts of these genes interact physically with Srb4, The recessive supp ressors include NCB1 (BUR6), NCB2, NOT1, NOT3, NOT5, and CAF1, which e ncode subunits of NC2 and the Not complex. NC2 and Not proteins are ge neral negative regulators which interact with TATA box binding protein (TBP), Taken together, these results suggest that transcription initi ation involves a dynamic balance between activation mediated by specif ic components of the holoenzyme and repression by multiple TBP-associa ted regulators.